Revert "Revert "Use __builtin_clz() to implement log_1()""
Revert "Revert "Use __builtin_clz() to implement log_1()""

This reverts commit 546f24e4f8a7c086b1e5afcdda624176610cbcf8.

And adds a fix for Windows: we need to use __builtin_clzll() rather than
__builtin_clzl(), because StgWord is unsigned long long on Windows.

ezyang/ghc,vTurbine/ghc,ezyang/ghc,vTurbine/ghc,mettekou/ghc,sgillespie/ghc,sgillespie/ghc,ezyang/ghc,vikraman/ghc,mettekou/ghc,sdiehl/ghc,olsner/ghc,snoyberg/ghc,snoyberg/ghc,sdiehl/ghc,vTurbine/ghc,vikraman/ghc,shlevy/ghc,sgillespie/ghc,shlevy/ghc,snoyberg/ghc,mettekou/ghc,snoyberg/ghc,shlevy/ghc,ezyang/ghc,snoyberg/ghc,vikraman/ghc,snoyberg/ghc,sgillespie/ghc,ezyang/ghc,mettekou/ghc,vikraman/ghc,sgillespie/ghc,vTurbine/ghc,shlevy/ghc,vikraman/ghc,mettekou/ghc,vTurbine/ghc,vikraman/ghc,mettekou/ghc,olsner/ghc,snoyberg/ghc,olsner/ghc,sdiehl/ghc,olsner/ghc,sgillespie/ghc,sgillespie/ghc,sdiehl/ghc,vTurbine/ghc,olsner/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,olsner/ghc,ezyang/ghc,ezyang/ghc,vTurbine/ghc,sdiehl/ghc,shlevy/ghc,vikraman/ghc,mettekou/ghc,olsner/ghc